1. Field of the Invention
The invention relates to industrial fabrics in general. In particular, the invention relates to fabrics used in the forming, pressing, and drying sections of a paper making machine and a method of manufacturing the same.
2. Description of the Prior Art
Industrial fabric means an endless fabric or belt such as one used as a forming fabric, press fabric, dryer fabric or process belt (“paper machine clothing”). It can also be a belt used as an impression fabric, TAD fabric, engineered fabric, a fabric used in the production of nonwovens by processes such as melt-blowing, spun bonding, hydroentanglement or a fabric used in textile finishing processes.
In general, during the papermaking process, for example, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in a forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The present invention is particularly advantageous with regard to press fabrics used in the press section. Press fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to support and to carry the paper product being manufactured through the press nips.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fill this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Contemporary press fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needled a batt of nonwoven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of the synthetic polymeric resins, such as polyamide, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or they may be flat woven using one or more layers of machine direction (“MD”) and cross-machine direction (“CD”) yarns, and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the MD yarns thereof. In this process, the MD yarns weave continuously back-and-forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a papermachine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing one base fabric within the endless loop formed by another, and by needling a staple fiber batt through both base fabrics to join them to one another. One or both woven base fabrics may be of the on-machine-seamable type.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce press fabrics, and other paper machine clothing, to the dimensions required to fit particular positions in the paper machines of their customers and therefore each fabric must typically be made to order.
In response to this need to produce press fabrics in a variety of lengths and widths more quickly and efficiently, press fabrics have been produced in recent years using a spiral winding technique disclosed in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al. (the '656 patent), the teachings of which are incorporated herein by reference.
The '656 patent shows a press fabric comprising a base fabric having one or more layers of staple fiber material needled thereinto. The base fabric comprises at least one layer composed of a spirally wound strip of woven fabric having a width which is smaller than the width of the base fabric. The base fabric is endless in the longitudinal, or machine, direction. Lengthwise threads of the spirally wound strip make an angle with the longitudinal direction of the press fabric. The strip of woven fabric may be flat-woven on a loom which is narrower than those typically used in the production of paper machine clothing.
The base fabric comprises a plurality of spirally wound and joined turns of the relatively narrow woven fabric strip. The fabric strip, if flat woven, is woven from lengthwise (warp) and crosswise (filling) yarns. Adjacent turns of the spirally wound fabric strip may be abutted against one another, and the spirally continuous seam so produced may be closed by sewing, stitching, melting, welding (e.g. ultrasonic) or gluing. Alternatively, adjacent longitudinal edge portions of adjoining spiral turns may be arranged overlappingly, so long as the edges have a reduced thickness, so as not to give rise to an increased thickness in the area of the overlap. Alternatively still, the spacing between lengthwise yarns may be increased at the edges of the strip, so that, when adjoining spiral turns are arranged overlappingly, there may be an unchanged spacing between lengthwise threads in the area of the overlap.
A multiaxial press fabric may be made of two or more separate base fabrics with yarns running it at least four different directions. Whereas the standard press fabrics of the prior art have three axes: one in the machine direction (MD), one in the cross-machine direction (CD), and one in the z-direction, which is through the thickness of the fabric, a multiaxial press fabric has not only these three axes, but also has at least two more axes defined by the directions of the yarn systems in its spirally wound layer or layers. Moreover, there are multiple flow paths in the z-direction of a multiaxial press fabric. As a consequence, a multiaxial press fabric has at least five axes. Because of its multiaxial structure, a multiaxial press fabric having more than one layer exhibits superior resistance to nesting and/or to collapse in response to compression in a press nip during the papermaking process as compared to one having base fabric layers whose yarn systems are parallel to one another.
The fact that there are two separate base fabrics, on top of the other, means that the fabrics are “laminated” and each layer can be designed for a different functionality. In addition, the separate base fabrics or layers are typically joined together in a manner well known to the skilled artisan including, depending upon the application, as aforesaid the needling of batt therethrough.
As mentioned above, the topography of a press fabric contributes to the quality of the paper sheet. A planar topography provides a uniform pressing surface for contacting the paper sheet and reducing press vibrations. Accordingly, efforts have been made to create a smoother contact surface on the press fabric. But surface smoothness may be limited by the weave pattern forming the fabric. Cross-over points of interwoven yarns form knuckles on the surface of the fabric. These knuckles may be thicker in the z-direction than the remaining areas of the fabric. Consequently, the surface of the fabric may have a non-planar topography characterized with localized areas of varying thickness, or caliper variation, which may cause sheet marking during a pressing operation. Caliper variation can even have an adverse effect on a batt layer resulting in non-uniform batt wear, compression and marking.
Laminated press fabrics, specifically multiaxial fabrics, may have such caliper variation. Specifically, in the special case of a multiaxial fabric having two layers with the same weave pattern, localized caliper variation may be intensified. Therefore, a need exists for a press fabric with reduced caliper variation to improve pressure distribution and reduce sheet marking during operation.
Other forms of papermaking fabrics are disclosed in U.S. Pat. Nos. 5,916,421; 5,939,176; 6,117,274 and 6,776,878 to Yook, and U.S. Pat. Nos. 6,378,566; 6,508,278; and 6,719,014 to Kornett, whose teachings are incorporated herein by reference.
One of the purposes of the present invention is also to address some of the limitations of current seamed multi axial fabrics. Specifically, spiral winding of “narrow” strips of woven cloth, may introduce discontinuities at the interface of each strip width when forming the seam via interdigitated loops as taught in the prior art. These discontinuities include: a) missing or distorted loops at each bond, occurring along the seam, and b) migrating CD yarn ends or picks that protrude into the loop sight tunnel periodically, at a frequency depending on the angle of the spiral wind (angle between warp yarn axis of narrow cloth and machine direction of final structure). In addition, the spiral winding technique taught in the prior art starts with a structure with a dimension of W×2 L (referencing the final dimension on the paper machine). If woven, this structure of size W×2 L is comprised of the same starting material in terms of yarn density and weave pattern. It has been learned, however, that layers having the same yarn density, spacing and weave pattern create an interference or Moiré Effect or pattern in the final structure.
It should be noted that in the case of most laminated multilayer fabrics whether or not multiaxial, some characteristic interference or the Moire Effect may occur since yarn alignment between layers is not often perfect. In laminated press fabrics, those consisting of two or more base structures or layers, such fabrics exhibit the Moire Effect that is a function of the spacing and size of both MD and CD yarns. This Effect is magnified if the yarns are single monofilament yarns, especially as the diameter increases and count decreases. The Effect also exists in multiaxial fabrics since the orthogonal yarn system of one layer is not parallel or perpendicular to those of the other layers.
Multiaxial multilayer fabric structures have provided many papermaking performance benefits because of their ability to resist base fabric compaction better than conventional, endless woven laminate structures. The reason for this is that, in the case of, for example, a two-layer multiaxial laminate, orthogonal yarn systems of one layer are not parallel or perpendicular to those of the other laminated layer. However, because of this, the relative angle between the respective MD and CD yarn systems of each layer (i.e. layers 110 and 120) ranges in practicality from 1 to 7° offset. The effect of this angle is that it greatly intensifies the Moire Effect and could cause the planarity of the interfacial topography to deteriorate.
The present invention describes a fabric comprising a flat woven full width on-machine-seamable base fabric and a process of making thereof, which address the limitations associated with fabrics in the described prior art. The present invention, specifically, solves the problems associated with Moire effect, and further provides for a faster production method, which overcomes the drawbacks of endless weaving.